Electron tube



Jan. 26, 1954 H. D. DOOLITTLE 7 9 ELECTRON TUBE Filed Nov. 22, 1950 H IIO 20 I8 l6 1@ 2| FIG. l

2e 29 25 :2 I I 33 37 as FIG.2

INVENTOR HOWARD D. DOOLITTLE ATTORNE Patented Jan. 26, 1954 ATENT OFFICEELECTRON TUBE Howard D. Doolittle, Stamford, Conn., assignor to MachlettLaboratories, Incorporated, Springdale, Conn.,

a corporation of Connecticut Application November 22, 1950, Serial No.197,040

3 Claims. I

This invention concerns an electron tube employing a novel evacuationduct through a solid anode block. More specifically this inventionconcerns an exhaust duct which emerges at the active anode surface, thatsurface at which electrons emitted from the cathode impinge, therebymaking possible the use of an expansion compensation means whichminimizes the change in interelectrode spacing due to heating.

Direct pumping of small tubes, particularly those employed in cavitycircuitry, has offered the problem of locating an exhaust duct at somepoint where its terminating tubulation would not be subject to theconstant danger of mechanical breakage. The only part of the tubeenvelope sufuciently protected to permit location of the exhausttubulation was found to be on the anode block, and the place usuallyavailable on the external portion of the block was found to be on thetubes axis at a point frequently surrounded by cooling apparatus.

Although the problem of mechanical breakage of the exhaust tubulationwas solved by axial location, there remained the problem what paththrough the anode block should be taken by the exhaust duct. Whenexperiments with simple straight exhaust ducts which emerged at theactive anode surface were tried, the power loss and upset of tubecharacteristics encountered were prohibitive. Thus, since ductsterminating at the active anode were unsatisfactory, alternativesolutions became necessary. As many anode blocks use essentially all theplanar top of the block perpendicular to the axis of the block as theactive surface, it became common for ducts to emerge at such anodeblocks side walls. Consequently, many designs today employ lateralpassages beneath the active anode surface connected to an axial duct.

In tubes of planar geometry, because most passages of present exhaustducts necessarily terminate in side walls of the anode block, stillanother problem has recently arisen. It has been found that when tubesheat up, the interelectrode spacing between the anode and its prox1-mate electrode often tends to decrease. Thus a change in capacity occurscausing a resultant change in the frequency of operation. This problemhas become especially serious in view of the fact that uses for highfrequency tubes in particular are demanding greater and greaterfrequency stability. The primary cause of this change in spacing hasbeen the differential expansion between the anode terminal, often madeof Kovar, and the anode block, usually made of ("ill surface.

copper. Since it was once the practice to join the anode terminal to theanode block at or near the base of the block and remote from the activesurface of the anode, there was great opportunity for differentialexpansion between terminal and block. Recently Kovar shells have beenused as a reentrant connection means between the base of the anodeterminal and a level on the anode block much nearer the active anodeThis has helped to reduce the change in spacing by reducing theeffective length of the block whose expansion opposes that of theterminal. However, because the exhaust ducts have penetrated the sidewalls of the anode block, the amount of anode block above the Kovarshell has had to be substantial. Thus even with the improved reentrantKovar shell, frequency shifts from 6 to 15 me. at a base frequency of200 me. over a temperature change of 20 to 180 C. have been experienced.

It occurred to me that were it possible to .terminate an exhaust duct atthe active surface of the anode, the reentrant Kovar shell could beextended and attached at the active surface of the anode block.Accordingly, ,I conducted ex:- periment which have shown thatif theexhaust opening through the active anode surface is no greater indiameter than one-half the distance between the anode and its nearestadjacent electrode, there will be a minimum of disturbance of thepotential field within the tube and. a negligible loss of power. I thatit is possible to make where needed providing the area of the opening issmall (e. g. in the order of 10 per cent or less) compared with thetotal cathode area. When exhaust ducts of this description are used itis found that the number of electrons which impinge the anode within theexhaust duct is sulficiently small to be immaterial despite the factthat they are out of phase with the electrons striking the active anodesurface.

Accordingly it is the object of my invention to provide an electron tubestructure which will maintain essentially uniform spacing between a.planar anode and its adjacent electrode despite severe heating.

Another object of my invention is to provide a simple exhaust duct foruse with tubes employing an anode block. If the diameter of the exhaustduct is kept to less than half the interelectrode spacing between theanode and its adjacent electrode, it is possible to terminate this ductat the active anode surface. In some instances this diameter may beexceeded providing have also discovered this opening larger,

anode block emerges at the active anode surface.

Fig. 2 shows another electron tube partially in section in which asimilar exhaust duct is at a slight angle.

Referring to Fig. l, the drawing depicts in partial section a coaxialdiode. The cathode l shown therein is preferably of the oxide coatedindirectly heated type. It is supported on a foil collar H which is inturn afiixed within the tube to terminal 12. Another heater terminal(not shown) is commonly used within terminal 12 and forms, together withinsulation sealing it to terminal 12, part of the vacuum wall. Ring 13is joined to terminal l2 by means forming part of the vacuum envelope. Aglass ring it joins ring l3 to the tubular anode terminal ring i5.

' A copper anode block [6 is commonly used to conduct away heatgenerated by electron bombardment of the active anode surface. Areentrant' shell I! made of material having the same coefiicient ofexpansion as the anode terminal is joined to the anode terminal at itsbase. As isshown here the reentrant shell may be made of cup shape sothat its bottom [8 may serve as the active anode surface. In this case avacuumtight joint may be conveniently made along'the side walls at thetop of the anode block it between it and the reentrant shell H. An axialexhaust duct [9 emerges at the active surface of the anode in an opening29' having a diameter preferably less than one-half the interelectrodespacing. A metal seal 01f tubulation 2| sealed to anode block H5 atshoulder 22, which may be formed by counterboring, is able to withstandthe bombardment of those few electrons which 1 enter the exhaust duct.As the structure becomes heated, the expansion of the anode terminal l5and glass [4' will tend to compensate that of the reentrant' shell I! sothat the interele'ctrode spacing will remain unchanged.

Referring to Fig. 2, a coaxial triode is shown in partial section.Within the tube cathode 25 is afiixed by foil tube 26 to terminal 21.The vacuum wall'may be completed by another heater terminaladvantageously sealed coaxially within terminal 21. Cathode terminal 21is joined to grid terminal 28 by means, including insulation, formingpart of the vacuum envelope. Grid terminal 28 supports grid 29 throughtubular grid su'D'pOrtw Tubular grid'te'rminal 23 is insulated fromtubular anode terminal 3| by glass ring 32.

The top of anode block 33 here serves as the active anode surface.Reentrant tubularmember 34 is afiixed' to thetop of the anode blockadjacent the active anode surface. end" of the reentrant member 34';whose coeflicient of expansion matches that of the anode terminal 3|, isafiixed to this terminal. a glass seal 35 is made at the end oftubulation The other 65 Because 36, the exhaust duct 31 which is againmade through the active anode surface is placed at an angle with theaxis so that electrons which enter may not cause destruction of theglass which is unable to conduct away heat as readily as metal. Thetubulation 36 may be brazed to the anode block at shoulder 38.

My invention is not confined-to the specific embodiments described butmay be applied with variations to any vacuum tube construction employingblock anodes.

I claim:

1-. In an electron tube having planar anode and cathode electrodeswithin a vacuum envelope, an anode end portion of coaxial constructionfor said electron tube comprising an anode terminal ring, a solid anodeblock, a member forming areentrant'part of the tube envelope andextending. between the anode terminal and the axial level of the activeanode surface at which level it is fixed to the anode block, and a ductwhich passes through the anode block and emerges at the planar activeanode surface thereby' reducing the area of said anode surface by anamount which is small compared to the cathode' area, the opposite end ofsaid duct terminating in'seal-off means.

2. In an electron tube having planar anode and cathode electrodes withina vacuum envelope, an anode end portion of coaxial construction for saidelectron tube comprising an anode terminal ring, a solid anode block, atubular member forming a reentrant part of the tube en'- velope andextending between the anode termi-' nal and the axial level of theactive anode sur-" face at which level it is fixed to-the' anode blockin a joint extending along the edge of the gen erally cylindricalsurface adjacent the active anode surface, and a' duct which passesthrough the anode block andemer'g'es at thejplanar anode surface therebyreducing the area of said anode surface by an amount which is smallcompared to the cathode area, the opposite end of said duct terminatinginseal-off means.

3. In' an electron tube having planar anode and cathode electrodeswithin a vacuum envelope, an anode end portion of coaxial constructionfor said electron tube'comprisin an anode terminal ring, a' cup memberextending" reen trantly into thetubeso that the planar bottom of said0111) provides the" active'anode surface, a solid anode block fixed tothe opposite surface of the bottom of said cup, and'a duct which passesthroughthe anode block andthrough the bottom of the'cupto emerge at'the'active anode surface therebyreducing the area of said anode surfaceby an amount which is" small compared to the cathode area, the oppositeen'd'of said duct t er minating in seal o'ff means;

HOWARD Di DOOLITTLE'.

dormley'et'al oct: 24'; 1950'"

